1. Field of the Invention
The present invention relates to a receiver of digital broadcasting such as digital audio broadcasting.
2. Description of the Related Art
In Europe, the DAB (digital audio broadcasting according to the Eureka 147 standard) is now in operation. In the DAB, a plurality of digital data are subjected to various encoding processes and finally converted to an OFDM (orthogonal frequency division multiplexing) signal, which is transmitted. Digital data such as digital audio data of up to 64 channels (maximum case) can be broadcast simultaneously.
FIG. 1(A) shows a time-axis structure of the OFDM signal. The OFDM signal is a plurality of continuous frames, and each frame consists of a plurality of symbols. The DAB has four transmission modes, that is, modes I-IV. The frame time length TF and the maximum number n of symbols in each frame of each transmission mode are as shown in FIG. 2.
Each frame is divided into a synchronization channel SC, a fast information channel FIC, and a main service channel MSC with a symbol serving as a unit of data. The synchronization channel SC, which is used for processes such as frame synchronization and AFC (automatic frequency control) in a receiver, consists of two symbols in which the first symbol is a null symbol Null and the second symbol is a phase reference symbol TFPR. The null symbol Null in every other frame includes identification information TII for identification of a transmission facility and no information is transmitted (no carrier signal is transmitted) during the other null symbol Null periods (every other frame).
In the fast information channel FIC, which provides data relating to the main service channel MSC and other data, data such as time, a date, a type, a data arrangement, and a traffic message control are arranged. In the main service channel MSC, digital audio data as main data and various kinds of digital data are arranged.
Further, as shown in FIG. 1(B), each symbol is configured in such a manner that a head period TG having a prescribed length is a guard period (also called a guard interval) that follows the preceding symbol and the remaining period is a data period of effective symbols. The contents of the period TG are the same as the contents of a tail period TE of the same symbol. Therefore, the periods TG and TE have the same length. The symbol time length TS and the length of the guard period TG of each transmission mode are as shown in FIG. 2.
As described above, the DAB has four transmission modes, that is, modes I-IV. One method for judging a transmission mode is a method of correlating a received phase reference symbol TFPR with another phase reference symbol TFPR that is prepared in advance and evaluating a correlation result.
However, in the case of this method in order to extract a phase reference symbol TFPR from a received signal, it is required that a rough sync is extracted from the received signal in a suitable timing. The rough sync is a synchronization signal of low precision that controls demodulation timing or the like. Further, since the phase reference symbol TFPR immediately follows the null symbol Null, there may occur an event that an attempt of extracting a phase reference symbol TFPR correctly and evaluating it results in a failure because a level of the TFPR symbol is influenced by AGC depending on the state of radio waves, for example, in a high field strength situation.
Still further, since only one phase reference symbol TFPR exists in each frame, one frame period is needed for each evaluation attempt and hence the transmission mode judgment takes a long time.
The present invention has been made to solve the above problems in the art, and an object of the invention is therefore to make it possible to judge a transmission mode quickly and reliably in a DAB receiver.
According to a first aspect of the invention, there is provided a digital broadcast receiver for receiving a digital signal that was transmitted in one of a plurality of transmission modes having different symbol time lengths and in which each symbol includes two signal intervals having the same period, comprising a delay circuit for delaying a received digital signal by a prescribed period; a correlation circuit for correlating a delay output of the delay circuit with the received digital signal; a moving average circuit for calculating a moving average of a correlation output of the correlation circuit over a period that is equal to the period of the two signal intervals; means for assuming a transmission mode to be one of the plurality of transmission modes in order; means for setting a delay time of the delay circuit to a time corresponding to the symbol time length of each assumed transmission mode; means for counting, for each assumed transmission mode, the number of times that an output of the moving average circuit exceeds a predetermined value; and means for judging, based on counting results of the counting means, that a transmission mode that gives a maximum number of times that the output of the moving average circuit exceeds the predetermined value is an actual transmission mode.
The count value becomes largest when the assumed transmission mode coincides with the actual transmission mode. Therefore, the assumed transmission mode giving the maximum count value is judged to be the actual transmission mode.
According to a second aspect of the invention, there is provided a digital broadcast receiver for receiving a digital signal that was transmitted in one of a plurality of transmission modes having different symbol time lengths and in which each symbol includes two signal intervals having the same period, comprising a delay circuit for delaying a received digital signal by a prescribed period; a correlation circuit for correlating a delay output of the delay circuit with the received digital signal; a moving average circuit for calculating a moving average of a correlation output of the correlation circuit over a period that is equal to the period of the two signal intervals; a peak detection circuit for detecting a temporal position of a peak of an output signal of the moving average circuit; means for assuming a transmission mode to be one of the plurality of transmission modes in order; means for setting a delay time of the delay circuit to a time corresponding to the symbol time length of each assumed transmission mode; means for judging an actual transmission mode based on an interval between peak temporal positions detected by the peak detection circuit.